This invention relates to embossing rolls or engraved rolls for tissue or plastic film or other webs, and, more particularly, to an embossing roll with removable embossing plates.
Paper products such as bathroom tissue and kitchen towels are commonly formed on a rewinder line in which one or more jumbo rolls of webs are unwound, perforated, and rewound into retail sized rolls. Many rewinder lines include an embosser for forming embossments in one or both of the webs and perhaps a glue deck to bond webs together.
The embosser conventionally includes one or more embossing rolls having an embossing pattern and a cooperating backup roll which presses against each embossing roll. The cooperating roll can be, for example, a meshing steel or paper roll or a compliant, smooth rubber-covered roll. A paper roll is formed from compressed paper or cloth. Steel and paper cooperating rolls are formed with recesses which mesh with the projections on the embossing roll. Each web is advanced between an embossing roll and its cooperating roll, and the embossing pattern is embossed into the web.
In most present commercial embossers, the embossing roll is manufactured integrally. That is, a roll body with journals is fabricated, and then the outer surface of this roll is engraved with an embossing pattern, commonly using acid and a resist, and/or indentation by a patterned tool. The problems with an integral embossing roll relate to cost and changeover time:
1. To get a new pattern, it is necessary to create an expensive new roll body.
2. To replace an old pattern, the heavy and expensive roll must be taken out of commission and shipped, for machining to a smaller size, and re-engraving.
3. Damage or wear in a limited surface region requires replacing the entire pattern.
4. When switching embossing rolls to produce a few days, worth of product with a different pattern, the exchange of rolls takes a considerable amount of time, perhaps longer than a working shift.
Covering a smooth precise roll with a removable (slightly undersized) sleeve the surface of which bears an engraved pattern is common in the printing art. It is also known in the embossing art; see, for example, U.S. Pat. No. 6,173,496 and EP 0 836 928 A1. However, this approach has at least several disadvantages:
1. Fabrication of sufficiently well-fitting steel engraved sleeves has been difficult, so printing technology has been used to make the sleeves, e.g., fiberglass sleeves, covered with hard nitrile rubber and laser-engraved. For ordinary production, these sleeves are not considered to be durable enough to be worth the expense.
2. A durable steel sleeve, thick enough for deep engraving, is very difficult to expand temporarily for installation on, and removal from, the supporting roll. In particular, the conventional compressed-air xe2x80x9cflotationxe2x80x9d method of Miller Graphics U.K., Ltd., Stork Screens America, Inc., Charlotte, N.C. or Strachan and Henshaw Machinery, Inc. is inadequate.
3. Removal of an entire sleeve can be accomplished quickly only if the embosser was designed specifically to support the roll body in a cantilever fashion, i.e., to hold a heavy roll at one end only, with clearance for the sleeve to be withdrawn over the other end. Furthermore, there must be enough space beside the machine to withdraw the entire length of the sleeve.
The advantages of removable plates have been recognized. For example, according to Leanna U.S. Pat. No. 4,116,594, when rolls are used to apply a continuous embossed pattern to a web, removable plates reduce the cost of pattern repair or replacement, and they also reduce the downtime of a changeover. However, all previous embodiments of removable plates:
have been slow and even difficult to change;
or were not firmly and uniformly preloaded against the roll;
or required a special embosser construction;
or were too large/heavy for a person to carry conveniently;
or interrupted the engraved surface with noticeable gaps;
or would not work with the thick plates required for deep engraving commonly practiced in this field.
The invention provides an embossing roll with embossing plates which are removably secured to a roll body. The removable plates provide the following advantages:
1. When changing embossing patterns, only the surface, i.e., the plates, is changed, not the entire roll body. Therefore, less investment is needed, and storage/shipping costs are reduced. This makes it feasible for converters to stock alternate or backup engraved patterns, and to take on smaller jobs, profitably.
2. The plates can be made of steel so that there is no sacrifice in durability.
3. Small gaps between plates accommodate thermal expansion and manufacturing inaccuracies better than a sleeve.
4. The plates are held to the roll with a fixturing system of vacuum suction and/or mechanical devices. Because the engraved surface is not in sleeve form, it is possible to attach/remove it from a roll without cantilevering that roll or removing it from the embosser (and without requiring substantial side clearance).
5. If the fixturing system includes quick-change features, it will be possible to change embossing patterns in minutes rather than hours.
6. There is no need to invest in a new embosser to utilize the removable plates. The invention will retrofit easily to most existing embossers.
The removable plates may be made of any sufficiently durable material. A key requirement is to provide means to hold them accurately, firmly, and safely against the surface of a fast-turning roll, while they are being pressed against a cooperating roll (which creates heat and xe2x80x9ccreeping tendenciesxe2x80x9d). Any holding method should permit reasonably quick changes and advantageously ensure safety in case power or vacuum is lost.
One embodiment uses vacuum to hold the plates, locating pins to guarantee precise location and prevent creeping (unimportant in some applications), and quick-change mechanical interlocks to retain the plates safely when vacuum is turned off. Other embodiments use vacuum alone or omit vacuum and use only mechanical attachments.
Vacuum holding of embossing plates was tested successfully on the rolls of a nested laminator, but it was recognized that customers might not find vacuum attractive for a mill environment (for reasons of contamination, maintenance, and maybe system cost). The preferred embodiments therefore use a purely mechanical plate-locking system.
To achieve the highest radial precision of the mounted-plate surface (essential for consistent glue application in laminated paper towel, and to prevent fretting), substantially the entire back surface of each plate is loaded firmly against the roll surface. But instead of using atmospheric pressure, the preferred embodiments achieve this by pulling tangentially at the edges of the plates, much as laces pull shoes tight on a foot. For low-precision applications such as rubber-to-steel embossing, a simple radial pull-down at multiple points could be effective.
Unlike magnetically held plates which must be flexible for easily peeling them off or on, the plates of the invention can be thick enough to permit deep engraving (even exceeding 0.070 inch depth).
To minimize pattern interruptions in continuous-web embossing, the invention involves inter-plate gaps smaller than 0.030 inch (perhaps even smaller than 0.010 inch), and all plate-fastening is effected from the plate underside. While some prior art stiff-plate die-changing has already involved underside fastening, it is not quick-change (especially on a long roll), and often requires substantial roll-end clearance.
The invention is quick-change: it permits securing or releasing an entire row of plates by means of just one or a few actions performed at the side of the embosser.
The invention does not rely on a single sleeve or even a series of short sleeves, because that would make it necessary to support the roll as a cantilever (i.e., support it by one end) or even remove it, while making a sleeve change. Nor need plates be slid axially to be removed, which requires both end clearance and the prior motion of other plates in the row. Instead, the plates may be removed transversely of the roll, a direction where there are few or no obstructions (rather than axially of the roll, where there is always a substantial obstruction), while the roll remains in place and supported at both ends.